Java Design Principles

A comprehensive guide providing essential design principles and best practices for Java development. Structured for step-by-step learning from beginners to advanced developers.

When to Use This Skill

Use this skill in the following situations:

• 🏗️ When designing new Java projects - Apply correct principles when making architectural decisions
• 🔧 When refactoring existing code - Check guidelines for code quality improvement
• 👀 When performing code reviews - Systematically review for design principle violations
• 🎯 When making class design decisions - Inheritance vs composition, interface segregation, etc.
• 🧩 When managing dependencies between modules - Methods to reduce coupling and increase cohesion
• 📚 When training junior developers - Systematically convey core design principles
• 🐛 When resolving technical debt - Identify which principles have been violated and improve

📚 Learning Path

🌱 Beginner - Starting with Simplicity

An introductory course for those learning design principles for the first time.

1. Simplicity Principles

   • KISS (Keep It Simple, Stupid)
   • YAGNI (You Aren't Gonna Need It)
   • Do The Simplest Thing That Could Possibly Work

   💡 Why learn this first? Before learning complex principles, developing the habit of writing simple code is most important.

🚀 Intermediate - Core Design Principles

Learn the core principles of object-oriented design.

2. SOLID Principles

   • Single Responsibility Principle
   • Open/Closed Principle
   • Liskov Substitution Principle
   • Interface Segregation Principle
   • Dependency Inversion Principle

   💡 Why is this important? SOLID is an essential principle for writing maintainable and extensible code.

3. Coupling and Cohesion

   • DRY (Don't Repeat Yourself)
   • Separation of Concerns
   • Minimize Coupling / Maximize Cohesion
   • Law of Demeter
   • Composition Over Inheritance

   💡 Practical Application: Learn how to properly design relationships between modules.

🎯 Advanced - In-depth Principles and Patterns

Learn more in-depth design principles and practical patterns.

4. Encapsulation

   • Encapsulation & Information Hiding
   • Encapsulate What Changes

5. Advanced Principles

   • Code For The Maintainer
   • Boy-Scout Rule
   • Avoid Premature Optimization
   • Inversion of Control
   • Command Query Separation
   • Robustness Principle (Postel's Law)

6. Practical Patterns

   • Good vs Bad examples of 5 core patterns
   • Single Responsibility
   • Open/Closed
   • Law of Demeter
   • Composition Over Inheritance
   • DRY

📖 Additional Resources

• Best Practices & Common Pitfalls

  • Checklists for design/coding/maintenance
  • 8 common mistakes to avoid

• Resources

  • Essential books
  • Online resources
  • Practice materials

🎓 Learning Tips

Recommended Order for Beginners

1. Week 1: Read and practice simplicity principles
2. Week 2-3: Learn SOLID principles one by one (one per day)
3. Week 4: Understand coupling/cohesion concepts
4. Week 5: Practice with practical patterns
5. Week 6+: Code review with Best Practices checklist

For Those with Experience

• Learn selectively starting with principles of interest
• Read Common Pitfalls first and apply to your current code
• Use this guide as reference documentation during team code reviews

📂 Project Structure

java-design-principles/
├── SKILL.md (this file)
├── core-concepts/
│   ├── simplicity-principles.md
│   ├── solid-principles.md
│   ├── coupling-cohesion.md
│   ├── encapsulation.md
│   └── advanced-principles.md
├── patterns/
│   └── common-patterns.md
├── best-practices.md
└── resources.md

🤝 Contributing

If you want to contribute to this guide:

• Open an issue if you find typos or improvements
• Send a Pull Request if you have better examples
• Share practical use cases

License: MIT Last Updated: November 2025

Happy Coding! 🚀